The demands of venue lighting are unique. For example, NFL stadiums generally light the field with a minimum of 250 foot candles at any point on the playing surface. To achieve this level of illumination with metal halide lamps requires roughly one megawatt of electrical power for the field alone. While metal halide lamps are presently the standard, they are not without drawbacks.
One concern with metal halide (also known as high intensity discharge, or HID) lamps is bulb life. While lower wattage bulbs may exhibit as high as 20,000 hour bulb life, higher power bulbs, such as the 1,500 watt bulbs commonly found in stadium fixtures, typically have bulb life expectancy in 3,000 hour range. A number of other concerns are related to bulb life, such as: envelope failure (bulb explosion) occasionally occurs towards the end of life or during bulb changes; lumen maintenance (brightness fall-off); cycling where the bulb turns off and on, seemingly at will; etc. While envelope failure is not common, it is of major concern since the envelope is made of glass and fixtures must enclose the bulb in such a way that flying glass cannot escape. Regardless, bulb failures in a fixture mounted on a tower high above a stadium are expensive and unwanted. To avoid catastrophic failures, many metal halide bulb manufacturers recommend group re-lamping at the end of the stated life, rather than spot changing individual bulbs.
Another concern is start-up and hot restrike. In a conventional probe-type metal halide bulb, ignition of a cold bulb involves igniting a small starter arc which brings the gasses in the bulb up to pressure and heats the gasses so that they are more easily ionized to start the main arc. This process typically take five to seven minutes, during this time the bulb produces significantly less light and the color temperature fluctuates significantly. Newer pulse start bulbs eliminate the probe and warm up times are reduced, but warm up can still take on the order of two to four minutes. While 1,500 watt pulse start bulbs and ballasts are available, they have not been widely accepted for field lighting, generally speaking, pulse start technology has found favor in lower wattages.
Hot restrike is of greater concern than initial start-up. Probe-type bulbs in the wattage range used for field lighting will not restart when the gasses in the bulb are hot. The hot restrike process can take up to 20 minutes. This problem was brought to the world's attention during the Superbowl in February 2013 when a momentary loss of power resulted in a 45 minute blackout during the game. Pulse start bulbs similarly reduce hot restrike times but the time delay required to reignite a bulb are still measured in minutes. Instant restrike ballasts are available for pulse start bulbs, but voltages on the order of 30,000 to 40,000 volts are required to restrike a hot 1,500 watt bulb. These voltages limit the distance between the bulb and the ballast and require special wiring with very high dielectric strength insulation to avoid arcing outside the bulb during a hot restrike.
Another concern in using metal halide bulbs is video production. Obviously video production of sporting events is a concern at the professional and college level, but video streaming has brought these concerns to even the high school level. While the broad spectrum nature of metal halide bulbs is generally good for video production, the light is not optimum for televising sports. For example, all metal halide bulbs are driven with alternating current. This means the arc reverses at twice the operating frequency. In the United States, a metal halide bulb, with a magnetic ballast, will flicker at 120 Hertz. If high frame rates are employed for slow motion, this flicker will be obvious in the final video. While high frequency electronic ballasts reduce the effect, it still exists.
Another issue for video production is the color rendering index (“CRI”) of the light. A simplistic definition of CRI is the percentage deviation between a light source and sunlight, but the effect is the ability of the light source to render colors. Skin tones are especially problematic for low CRI light sources. The metal halide bulbs used in sports complex lighting typically have a CRI of about 65. While the light produced by such bulbs usually appears very white, the light typically has a surplus of energy in the 500 nm range of the spectrum, or a green spike. A green spike, coupled with green light bounce off the field, is typically handled by “white balancing” the cameras, but is still less than ideal for professional video production.
Yet another concern with metal halide bulbs is the production of ultraviolet light (UV). These bulbs produce significant amounts of short wave UV which can be dangerous to humans. Most bulbs include a borosilicate or fused silicate outer envelope which will absorb the vast majority of the short wave UV light. If the outer envelope is broken, most metal halide bulbs will continue to function but will emit dangerous amounts of UV light. So called “flash burns” or sunburn of the eye is a real danger to people in proximity to such bulbs. Even with the outer envelope in place such bulbs emit enough UV light to be damaging to plastics and can cause some finishes to fade over time.
Finally, there are environmental concerns with the disposal of such bulbs, in particular due to the use of mercury. While manufacturers have found ways to reduce the amount of mercury used in metal halide bulbs, some mercury is required to produce white light. Since the bulb envelope is glass, breakage after disposal is likely and thus the release of mercury is likely.
Light emitting diodes (LEDs) offer improvements over metal halide bulbs in all of these areas. However, light emitting diodes are not without their own challenges. Perhaps the biggest challenge to producing an LED luminaire for venue lighting is thermal management. A metal halide bulb radiates close to 85% of the input power as visible light, ultraviolet light and infrared energy, leaving 15% of the power which must be dissipated into the environment through conduction. In contrast, an LED radiates virtually no ultraviolet light and virtually no infrared energy, thus at least 55% of the input power must be dealt with through conduction. This is particularly problematic with large arrays of lights where hot air from lower fixtures in the array effectively raises the ambient temperature around higher fixtures.
LEDs are finding their way into indoor venue lighting. Such lights offer the advantage of instant on, whether hot or cold, and are even full range dimmable, unlike their metal halide counterparts. Indoor fixtures, of course, do not have to accommodate a wide range of ambient temperatures. Indoor venues can easily employ larger numbers of lower power fixtures, which can be located directly above the playing surface. Further, indoor fixtures do not have to compete with daytime light levels.
Some attempts have been made at lighting outdoor venues with LED fixtures. To date, such fixtures have been very large compared to metal halide fixtures or produce far less light for a comparable form factor. This would be particularly problematic in retrofitting towers in existing venues which have metal halide fixtures. Regardless, in both indoor and outdoor attempts, these fixtures have employed one lens for each LED or module, all employ multiple lenses. All of these lights will exhibit an inverse square fall off the light when the light strikes the playing surface at an angle and not straight-on. Typically these lenses have a relatively short focal length making it difficult to manufacture a fixture with consistent focus from LED-to-LED. The result is a bright hot-spot in the middle of the beam. Thus, to achieve very even lighting of the field is very difficult, at best.
Finally, neither metal halide lamps nor existing LED fixtures are particularly dark sky friendly. A movement has been afoot for several years to reduce unwanted light spillage into the night sky, or “light pollution.” Many outdoor metal halide fixtures include an “eyebrow” or visor to reduce the amount of upward spillage. This is only marginally effective. Metal halide bulbs emit light spherically. Only a small portion of the produced light is emitted toward the field. Fixtures typically use an aluminum reflector to capture some of the light headed rearward and reflect and focus it toward the field. A little more than one-third of the light produced by the bulb actually makes it to the intended target. Even with the visor, a significant portion finds its way skyward.
Individual LEDs are typically packaged to emit nearly all of the produced light in a forward direction. The types of LEDs currently employed in venue lighting typically emit light in a 120 degree beam. Most known fixtures use multiple small molded lenses, often called TIR lenses, to capture virtually all of this light and focus it into a narrower beam. Unfortunately, these fixtures also then employ a second clear lens to protect the LEDs and molded lenses from the elements. Some of the light striking this lens is reflected rearward into the fixture and later reflected back out of the fixture in random directions, including skyward.
Many outdoor architectural light fixtures, as well as other large outdoor area lighting fixtures, suffer from these same problems. In particular, inverse square fall off and dark sky issues are problematic in metal halide fixtures used to wash building walls, in fixtures used for airport tarmac lighting, etc.
Thus there is a need for a high power stadium outdoor light fixture which will minimize lamp replacements, is not constrained by a restrike interval, provide video friendly light, minimizes emissions outside the visible light range, provides effective thermal management, will not fail explosively, and minimizes skyward light emissions.